Higher density recording is now required for magnetic recording media, and one approach of smoothing the surface of a magnetic layer is known.
As a result of smoothing the surface of a magnetic layer, however, the friction coefficient of contact between the magnetic layer and running system increases while a magnetic recording medium runs, damaging the magnetic layer or causing it to peel apart in a short period of time.
Particularly, a video tape is often used under harsh conditions, such as in a still mode. In this case, ferromagnetic particles readily separate from a magnetic layer, causing head gap bridging.
In order to improve the running durability of a magnetic layer, it has been suggested to add abrasive agents (hard particles) such as corundum, carbon silicate or chromium oxide to a magnetic layer, although a relatively large amount thereof must be added to provide abrasive effects.
However, the addition of a large amount of abrasive agents is unfavorable, since such a magnetic layer having a large amount of abrasive agents causes remarkable magnetic head wear and has reduced smoothness, which is undesirable since a magnetic layer is smoothed to improve electromagnetic properties.
It has also been suggested in U.S. Pat. No. 4,322,474 that a fatty acid or an ester of a fatty acid and an aliphatic alcohol be added to a magnetic layer with lubricating agents to reduce its friction coefficient.
As use of portable video tape recorders and flexible disk drive apparatuses for a personal computer has spread recently, magnetic recording media are increasingly used under various conditions, such as low temperature or high temperature and high humidity. Accordingly, the running durability of a magnetic recording medium should not fluctuate, but should remain stable even under various conditions. Conventionally used lubricating agents are insufficient to provide such running durability.
With the rapid reduction in size of magnetic recording media, particularly video tapes and floppy disks attained by compressing recording wavelengths and track width, smaller ferromagnetic alloy particles are used more often than ferromagnetic particles of iron oxide type, conventionally used as materials for magnetic particles.
As magnetic particle size is made smaller, comparatively better electromagnetic properties can be obtained, but it is difficult to simultaneously improve durability.
In order to solve the above problem, it has been suggested in Japanese Patent Application (OPI) No. 227030/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and in Japanese Patent Publication No. 46128/82 that as a lubricating agent, diethylene glycol ester derivatives be used in combination with binders of urethane type, vinyl chloride type or epoxy type.
The use of diethylene glycol ester derivatives, alone or in combination with olive oil, can improve durability at high temperatures, but .mu. values after repeated running at low temperature and low humidity increase and head gap bridging occurs at low humidity.
That is, such lubricating agents cannot maintain their lubricating effect at low temperature or at low humidity, and are insufficient with respect to electromagnetic properties such as S/N and magnetic properties such as squareness ratio.
Further, when these lubricating agents are used in combination, head gap bridging takes place at high humidity, and .mu. values (friction coefficient) increase after running repeatedly at room temperature.